We propose to do an analysis of the structure, replication, and genetic properties of yeast chromosome XII. The structure and recombination properties of the repeating ribosomal DNA (rDNA) genes, which are located on chromosome XII, will be examined in detail. The proposed research will be done in several steps: 1) Genetic and physical maps of the rDNA gene cluster will be constructed. This will be done by using the yeast transformation procedure to insert a selectable non-repeating yeast gene within the rDNA gene cluster. The position of these inserted genes will then be mapped by traditional genetic techniques. 2) From a large collection of recombinant clones containing insertions of yeast DNA, one or more clones that have non-repeating DNA from yeast chromosome XII will be selected. 3) This recombinant clone will then be used in DNA-DNA hybridization experiments to detect other recombinant clones which contain DNA from chromosome XII. By repeating this procedure several times with several different probes, a physical map of chromosome XII will be generated. 4) A number of loci (such as the DNA initiation sites, the centromere, etc.) will be located on the physical map and characterized.